To prevent this, a thin film of oil or similar waterproof coating is applied to avoid contact of pickled steel with moisture in the air.

The oil film must be removed for any further fabrication, plating or painting processes.

Inhibitors are natural or synthetic organic compounds that are added to acid solutions to protect plain carbon steels being cleaned.

They are added for slowing or stopping the chemical action of the acids on the base metal.

Inhibitors:

reduce the loss of iron,

minimize hydrogen embrittlement,

protect the surface from pitting attack caused by excessive pickling or from poor surface condition quality,

reduce fumes resulting from excessive reaction of acids on metal, and

help economize on acid consumption, without affecting appreciably the rate of scale or rust removal.

For alloy steels, uninhibited acid solutions are preferred because a more energetic chemical action is required to remove the oxides.

If the surfaces covered with scale and rust are generally and uniformly clean from additional contaminants and if the processing time is not critical, no other pretreatment is required.

Rusted or scaled surfaces submitted to cleaning that are contaminated by soils that do not react with acids, like grease, oils, soaps, lubricants and other coatings, must be precleaned by an additional process.

Usually alkaline cleaning is especially advantageous when acid immersion time is limited to a few seconds in a continuous working line.

Limitations of acid cleaning consist in the difficulty to handle the products because of their corrosiveness, and in the problem of hydrogen embrittlement that may significantly affect some alloy and high-carbon steels.

The hydrogen liberated from the acid reacts with the surface, makes it brittle and may cause cracks.

To assure desired scale removal rates without endangering these steels, acid concentrations and solution temperatures must be kept under stricter control.

Sludge is the waste product from this process, that includes acidic rinse waters, metallic salts and waste spent acid.

EPA considers all these as hazardous waste to be treated before being disposed of. Sludge from steel processes should be neutralized with lime before being discharged in a land fill.

Those engaging in these processes should adopt responsible attitudes, be familiar with environmental protection requirements and make sure to meet them always in their small or big industrial activities.

Watch the following Video on

Monterrey DBI Push Pull Pickling Line

http://www.youtube.com/watch?v=LjzxBDLsDrQ

* * *

Any questions or comments or feedback? Write them down and send them to us by e-mail. Click on the Contact Us button in the NavBar at top left of every page.

Let us remind you that, if you are interested, we offer a no cost subscription to our Practical Welding Letter and a bonus book in pdf format to be made available for download to your computer on the subject of
PRACTICAL HARDNESS TESTING MADE SIMPLE. Click on Subscription.

To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics.